The present invention relates to a pattern reading apparatus for reading a pattern formed on a surface of a silicon wafer or the like, and more specifically, to a pattern reading apparatus for reading a pattern formed on a reflective or transparent surface.
In manufacturing semiconductor products, semiconductor layers are applied to a semiconductor substrate, such as a silicon wafer or the like, by vapor deposition and then design patterns are formed by photo-lithography processes, etching processes, and the like. In general, a serial number is applied to the silicon wafer by laser etching so that the silicon wafer can be tracked during the pattern forming processes based on the serial number. Conventionally, the serial number on the silicon wafer is discriminated by a worker visually examining the wafer.
However, since the silicon wafer is mirror finished, for a worker to read the serial number, the wafer must be viewed obliquely while holding it to the light, or by some similar method. Further, since the quality of the pattern may deteriorate as the silicon wafer is subjected to processes such as etching, vapor deposition and the like, it is particularly difficult to discriminate the serial number of the silicon wafer after a number of such processes.
Conventionally, two types of pattern reading devices have been known: a reflective-type reading device, and a transmission-type reading device. The former is used for reading a pattern formed on a reflective surface, and the latter is used for reading a pattern formed on a transmission-type surface.
In an example of the reflective-type reading device, light emitted by a light source is incident, through a lens, to a surface on which the pattern is formed, and an image of the pattern is formed by an imaging lens on a screen or the like. In this case, a portion of the light incident to the lens is reflected on a surface of the lens to create ghosting light, which reaches the screen and reduces contrast of the image of the pattern. Further, the specular reflection from the surface having the pattern formed thereon may be incident on the screen making it more difficult to observe the image of the pattern.
As an example of the transmission-type reading device, a known device has a Fourier transformation lens, that is used for reading a pattern formed on a light-transmission-type object by subjecting the pattern to a predetermined filter processing. In these optical systems, the light beam from a point light source passes through a first lens and is incident on an object as a parallel light beam. After passing through the object, the light beam is converged by a second lens and caused to pass through a spatial filter disposed at the back focal point of the second lens. When an imaging lens, having the front focal point set to the position of the filter, is disposed behind the filter, an object image, which is affected by the function of the filter, is formed at the back focal point of the imaging lens.
For example, to output an emphasized image of a pattern formed on an object surface, a high-pass filter may be used as the spatial filter to shade the paraxial rays which correspond to the image of the point light source. Further, an imaging element may be disposed at the imaging position to capture and process the image for further processing or displaying on a display unit.
In the above conventional filtering optical system, however, when an objective lens (first lens) has spherical aberration such as, for example, a spherical single lens or when coma and curvature of field arise because a light beam is obliquely incident on the objective lens, there is a problem in that the light beam which forms the image of a point light source does not converge to a point but scatters over a larger area such that a large shading region must be provided to properly execute filtering. Thus, a quantity of light used to form the image is lowered.
In a pattern reading apparatus using the above conventional filtering optical system, since the magnification of a pattern image having been formed cannot be changed, the pattern image cannot be optically enlarged or reduced. That is, since an object surface is disposed to the focal point of an objective lens in the conventional optical system, the light beam emitted from the objective lens is made afocal. Thus, even if the imaging lens is moved, magnification cannot be changed. To change the magnification, the imaging lens must be composed of a group of a plurality of lenses.
Further, a pattern reading apparatus using the above conventional filtering optical system cannot be easily used when the object to be read is intended to function as a prism (i.e., has a wedge shape or the like) for deflecting a light beam. In this case, the image of the point light source will not be shaded by a spatial filter because the image will be formed at a position outside of the axis. Thus, a component of light other than the scattered reflected component will be incident on an imaging lens and a desired filtered output image cannot be output. A similar problem also may arise when a reflection surface is tilted at the time a pattern is read by this type of apparatus.